Two ocean states during the Last Glacial Maximum

Abstract

The last deglaciation is the best constrained global scale climate change documented by climate archives. Nevertheless, the understanding of the underlying dynamics is still limited, especially with respect to abrupt climate shifts and associated changes in the Atlantic meridional overturning circulation (AMOC). A fundamental issue is an appropri- ate climate state at the last glacial maximum (LGM, ∼ 21 000yr ago), which is used as initial condition for deglaciation. Using a comprehensive climate model, we show that for an identical set of LGM boundary conditions two different water mass configurations and associated AMOC states can coexist with respect to the salinity stratification in the deep Atlantic Ocean. Only one of the two ocean states is consistent with the available reconstructions, e.g. shallower AMOC and more expanded Antarctic Bottom Water. Furthermore, we also show that the salinity stratification represents a key control on the spatial configuration, the strength of the AMOC as well as the transient response of the AMOC to freshwater perturbation and therefore bears the potential to reconcile the apparent differences among models and data. In combination these findings represent a new paradigm for transient deglacial climate changes at the end of the last ice age that challenges the conventional evaluation of glacial and deglacial AMOC changes based on an ocean state derived from LGM boundary conditions.